Sudden Failure in Beam 1

[chocomocha]

Hi! Have anybody here encountered concrete design problem, where your steel is yielding and and you cannot increase the size of your concrete section anymore resulting to the most-dreaded "sudden failure"? Thank you very much.

 

[JAE]

Usually flexural failures in concrete beams are not sudden but, if under-reinforced per the codes, have a gradual failure with cracking as the warning.

Shear failures tend to be more sudden and therefore have the lower [φ] factors.

Can you say more about your situation?

 

[TXStructural]

It sounds like you are describing a compression failure in flexure. The beam is over-reinforced and the easiest solution is to add compression steel. This is basic concrete beam design.

 

[chocomocha]

The situation is, we have a beam that was designed using steel rebars, however, due to additional loadings as per client's request, we are now facing a dilemna.We are using high strength concrete (50Mpa), and our beam size is already controlled, so we cannot change our sizes already. Due to the additional loads, our beam cannot withstand the new Moment. Adding rebars is not a solution too since it is still failing, so we decided to use FRP. The problem is, we were able to satisfy the moment capacity, but we are over reinforced already. We are using ACI 440.Thanks.

 

[chocomocha]

We are actually designing it using internal glass fibres. I suggested that we use laminates instead, but my boss is firm with the glass fibres mixed in concrete. As far as I know, doing so, will not increase design strength, only the material strength.

 

[JedClampett]

This thread has me kind of scared. Is it steel reinforcing, glass fibers, laminates or what? You've got everyone confused.
Steel yield is assumed for concrete beam ultimate strength design. Just because the steel yields doesn't mean you have a failure (although you will get cracking), although you're best off staying away from it. Concrete crushing is the failure point.
It sounds like you need to tell the client that the beam was designed for a certain load and their new loads cannot be added and keep within code allowables. Making a client happy doesn't mean jumping through unrealistic hoops.

 

[TXStructural]

I'm with Jed. Sounds like a more experienced engineer needs to look at the problem.
Glass fiber will not strengthen the beam, it will only change the character of the concrete until it cracks. Then you have the steel reinforcing providing all the tensile strength for flexure and shear.

Bonded external FRP provides tensile strength. If the design meets ACI 440.2R-8 design guidance, then there should be no issue with "over-reinforced".

Consider higher strength reinforcing steel, but be aware that over 75/80 ksi (500/550 MPa) you could have serviceability issues (wider cracks or more deflection at higher elongation under load) as the tension force approaches capacity.

If the beam size is not negotiable, and you have maxed out the reinforcing in both the top and bottom of the beam, and have looked at redistribution of moments, then you need to look into significant redesign. I have very seldom seen a beam which can't be made larger in some direction.

You might also consider post-tensioning, internal or external.

 

[rapt]

As mentioned above, glass fibre is not viable in this situation.

If it is badly over-reinforced, then it is the same no matter what tension reinforcement type is used. FRP or prestress will not help you.

The only solution as suggested above is more concrete and depth is more efficient. Tell the client you need more concrete depth (and do not forget that you compression reinforceemnt must be fully tied as for a column, and as concrete strength increases these rules become much more onerous)!

 

[sdz]

You can get more moment capacity by using compression steel properly confined with ligatures (ties, stirrups) in accordance with the relevant provisions of your code.

You haven't really given enough information to know if this is a viable solution.

I don't think 50MPa is generally regarded as high strength these days.

 

[chocomocha]

Thank you all guys... We'll be meeting the client to discuss these issues. I really need to change my beam sizes. You are all of great help. I even recommend my boss to join this forum. This is very helpful.